This invention relates to an electrolytic capacitor and, more particularly, to an improvement in a chip-type electrolytic capacitor enclosed in a synthetic resin impervious to liquid.
With the recent development of ICs, electric and electronic components to be used therewith have been becoming smaller and smaller. For the electrolytic capacitor, a very small capacitor element has been manufactured by winding electrode foils each of, for example, a few milimeters in width into a cylindrical shape of 5 mm or less in diameter. In such case, the electrolytic capacitor element is normally moulded with a synthetic resin instead of being encapsulated in an aluminum case to form the final capacitor. Unlike other electronic components, however, an electrolytic capacitor requires special care because of its polar construction and impregnation with an electrolyte. That is, the mould of the electrolytic capacitor element with the synthetic resin must be performed liquid-tightly in order to keep its electrical properties such as capacitance constant. Specifically, since, with a passage of electric current through the electrolytic capacitor element, hydrogen gas is generated due to electrolysis of the electrolyte, the mould of the electrolytic capacitor must be kept airtight enough to withstand increase in inner pressure caused by generation of gas as well as to prevent evaporation of the electrolyte and contamination by foreign impurities.
In moulding the electrolytic capacitor element with a synthetic resin, the affinity of terminals or metal plates extending outwardly from the element with the resin is the key factor to the airtightness of the mould. When the miniaturization of the electrolytic capacitor element causes the metal terminal plates to be designed smaller in size, the affinity between the resin and the metal plates becomes more severe. Since thermoplastic resin is generally poor compared with thermosetting synthetic resin in its affinity with metals, the moulding with a thermoplastic resin can be poor in airtightness. In that sense, a use of thermosetting resin is preferrable. For the moulding with thermosetting resin, the curing temperature of thermosetting resin is as high as 150.degree. C. to 180.degree. C. Therefore, when the electrolytic capacitor element impregnated with electrolyte is moulded with the resin and heat-treated into the heated resin at such high temperature, the electrolyte evaporates. In order to avoid this, the impregnation with electrolyte should be performed after the moulding is completed. However, since the electrolytic capacitor element is moulded with thermosetting resin under a pressure of about 100 kg/cm.sup.2, the elements is compressed and tightened causing a subsequent impregnation with the electrolyte to be very difficult, resulting in that a desired capacitance of a resultant capacitor can not be obtained and dielectric dissipation factor (tan .delta.) thereof becomes too large. Furthermore, the compression may damage the oxide layer on the electrodes, resulting in an increase of leakage current. In addition, the resin mould may have a hole through which the electrolyte can be introduced to the element inside the mould. Such a hole is very hard to be closed after the introduction of the electrolyte, leading to a poor airtightness.